Optical switching, multiplexing, and demultiplexing have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with a second star coupler via an optical grating consisting of an array of optical waveguides. Each of the waveguides differs in length with respect to its nearest neighbor by a predetermined fixed amount. The outputs of the second star coupler form the outputs of the switching, multiplexing, and demultiplexing apparatus. See, for example, my U.S. Pat. No. 5,002,350 issued Mar. 26, 1991.
In operation, when each of a plurality of separate and distinct wavelengths are launched into a separate and distinct input port of the apparatus, they will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, a plurality of input wavelengths is directed to a predetermined one of the input ports of the apparatus. Each of the input wavelengths is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelength also permits switching between any selected input port to any selected output port.
The grating located between the two star couplers essentially consists of an array of curved waveguides of different lengths. The waveguides are closely spaced at their ends, whereas they are widely spaced and strongly curved in the central region. The order of the grating is determined by the difference in length between adjacent waveguides. For many applications, the order of the grating must be large, normally greater than 50. As a result, the grating then becomes large and is difficult to make with satisfactory accuracy, particularly when very low levels of cross-talk are desired. Actually, defects of fabrication will, in general, cause waveguide width variations which will affect the propagation constant in each arm of the grating, thus causing phase errors that will substantially increase cross-talk in a multiplexer. In addition, to keep bend losses to a minimum, the radius of the waveguide bend is limited.
It is, therefore, an object of this invention to reduce phase errors in grating waveguides which result from fabrication variations. It is also an object of this invention to reduce the bend loss to allow the bend radius of the waveguide bend to be further decreased without further increasing bend losses.